(1) Field of the Invention
The present invention relates to a magnetic core widely used in electronic equipment, and a coil assembly and a power supply circuit, each incorporating the magnetic core and, more particularly, to a technique for introducing compact, low-loss, simple-construction, high-efficiency and resource-saving designs into a magnetic core, a choke coil, and a transformer for use in a switching power supply.
(2) Description of the Related Art
To promote compact and high-efficiency designs, a switching technique has been introduced into power supplies. Insulator-type power supplies with a transformer incorporated therein have widely employed a single-ended forward DC-DC converter or a full-bridge DC-DC converter to obtain an output power much higher than 100 W.
Currently used switching power supplies have a reactive filter structure to comply with harmonic restriction rules which require that a drop in power factor due to distortions in currents and voltages to utility power source be controlled. Rather than using conventional choke-input rectification, the reactive filter structure rectifies a utility power input with a reactor at a subsequent stage grounded via a switching transistor. During an off state subsequent to the on state, electromagnetic energy stored in the reactor during the on state is released into an output capacitor. Thereafter, electromagnetic energy is fed to a DC-DC converter, which constitutes a main portion of the power supply.
The above-mentioned conventional art has the following serious problems in industrial applications.
When a conventional DC-DC converter providing a relatively large output is used, that employs the structure of a single-ended forward DC-DC converter to avoid a bulky structure. Accordingly, a smoothing choke coil is required to smooth the transfer of power during the on state and the off state of main transistors. This arrangement leads to a complex and less economical circuit design.
To obtain a high output power, a full-bridge structure is employed. In this way, the utilization of the transistors is heightened by taking advantage of the first quadrant and the third quadrant of the B-H characteristics of a transformer. Although a high output power results, four main transistors are required. Since two identical ones of the four transistors become concurrently conductive, the full-bridge structure suffers from substantial loss in economical and industrial points of view.
The conventional reactive filter circuit uses a reactor that is as bulky as an output transformer of the DC-DC converter at a subsequent stage connected to the reactor, and copper loss of the coil of the reactor lowers the efficiency of the entire power supply.
It is an object of the present invention to provide a magnetic core, a choke coil, and a transformer, used in a switching power supply, and featuring compact, low-loss, simple-construction, high-efficiency and resource-saving and economic designs.
It is another object of the present invention to provide a coil assembly employing the magnetic core.
It is still another object of the present invention to provide a power supply circuit employing the coil assembly.
According to one aspect of the present invention, there is provided a magnetic core which has an open magnetic path, and includes a magnetic body forming the open magnetic path and having a soft magnetic property, and a permanent magnet at least at one end of the magnetic path of the magnetic body.
According to another aspect of the present invention, there is provided a coil assembly which includes a magnetic core having an open magnetic path. The magnetic core includes a magnetic body forming the open magnetic path and having a soft magnetic property, and a permanent magnet at least at one end of the magnetic path of the magnetic body, and also includes at least one coil having at least one turn wound around the magnetic core.
According to still another aspect of the present invention, there is provided a power supply circuit which includes a coil assembly including a magnetic core having an open magnetic path. The magnetic core includes a magnetic body forming the open magnetic path and having a soft magnetic property, and a permanent magnet at least at one end of the magnetic path of the magnetic body, and also includes at least one coil having at least one turn wound around the magnetic core. In the power supply circuit, a magnetic field is generated in response to an exciting current that flows through the coil of the coil assembly when an input voltage is applied. The magnetic field is applied to the magnetic body having the soft magnetic property, and is opposite in direction to a magnetic field which is applied to the magnetic body having the soft magnetic property by the permanent magnet.